Calianna's post basically explains why I don't bother weighing and measuring anymore. Another point: the differences among the actual samples of food.

• Grow 4 or 5 varieties of tomatoes, taste exactly 15 g of each one, and try and tell me they have the same calorie count. The juicy, tart ones aren't the same as the dense, sweet ones.
• Look at the difference in marbling among different packages of the same cut of meat.
• Try n' tell me that a watery pale-yolk February grocery store egg has the same nutrition count as, say, a spring farm egg where the hens eat a completely different (probably better) diet.
• A while back, I bought some beautiful blueberries that tasted like crap. No sweetness and no blueberry flavour. If you ate them blindfolded, you wouldn't know what you were eating. There's no way they have the same nutrition count as "real" blueberries.

(Getting a bit off-topic, the other reason I don't count/measure anymore is that I cook with too much homemade yogurt and broth, which is never the same twice. If I felt the need to measure again, it would be an awful lot of guesstimating.)